Most battery powered computer systems, for example, laptop computers, personal digital assistants, mobile phones, pager devices and the like, have some type of display light to assist a user in viewing the display contents under conditions of inadequate ambient light. Such lights are often referred to as “backlights,” and less frequently as “frontlights.” In addition, some battery-powered computers, e.g., mobile phones, may illuminate a keypad as an aid to usage under less than ideal ambient lighting conditions. Examples of such light sources include cold cathode florescent tubes, electro-luminescent panels and light emitting diodes.
When operating, these light sources can oftentimes consume high levels of power from the device's limited battery capacity. As a consequence, some such battery powered computer systems are designed to detect ambient light levels, and to turn off device lighting if there is sufficient ambient light for a user to view the display (and/or keypad) without system lighting.
Conventionally, a silicon photodiode or silicon phototransistor has been coupled to an analog to digital converter in order to provide a measurement of ambient light levels to such battery powered computer systems. Silicon-based photo-sensitive devices are commonly available and are familiar to many practitioners of the electronic arts. Further, use of such silicon-based photo-sensitive devices is illustrated in many design “cook books” commonly used by engineers.
Unfortunately, silicon-based photo-sensitive devices generally do not match the response of the human visual system to varying sources and wavelengths of light. For example, a silicon-based photo-sensitive device will detect or measure approximately 5% of the light intensity from florescent lighting as will the human eye. As a deleterious result, such a silicon-based photo-sensitive light measurement system may falsely report that there is insufficient ambient light for viewing a display under florescent light, when in fact there is plenty of light for the human user. In such a situation, the computer system may unnecessarily provide display lighting, unnecessarily consuming limited battery power, and unnecessarily reducing the operating time of the battery powered computer system.
Likewise, for some common light sources, e.g., halogen lighting, silicon-based photo-sensitive devices are more sensitive to the light energy than the human eye. In such cases, the computer system may deleteriously determine that there is sufficient ambient light for viewing a display, when in fact there is not. The detrimental result is that the computer system may not provide display lighting, and the user will be unable to view the display.
A further disadvantage of many conventional systems is the common use of an analog to digital converter to functionally couple a light sensitive device to a processor. Analog to digital converters as commonly used in this application are typically slow, requiring a conversion cycle that is very long compared to the operating speed of a modern computer processor. The analog to digital converter frequently requires an external clock source to control the conversion process. These characteristics may require additional software complexity to initiate, control and receive a converted light measurement.
In addition, since the output of most analog to digital converters is a multi-bit value, e.g., typically from eight to 12 bits, accessing the value is complex. For example, if the value is accessed serially, a universal asynchronous receiver/transmitter (UART) or other serial interface port may need to be dedicated to the analog to digital converter. Alternatively, some analog to digital converters may be interfaced in parallel, requiring, e.g., eight to 12 additional wiring channels on a printed circuit board. As an undesirable result, using an analog to digital converter for this application suffers an increase in design complexity, cost, and consumes excessive development resources.
Thus a need exists for a method and system of ambient light detection with digitized output. A further need exists to meet the previously identified need that is complimentary and compatible with conventional computer system design techniques. In conjunction with the aforementioned needs, a still further need exists for detecting ambient light with sensitivity that closely matches the sensitivity of the human visual system.